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Since the first pair of BeiDou satellites was deployed in 2000, China has made continuous efforts to establish its own independent BeiDou Navigation Satellite System (BDS) to provide the regional radio determination satellite service as well as regional and global radio navigation satellite services, which rely on the high quality of orbit and clock products. This article summarizes the achievements in the precise orbit determination (POD) of BDS satellites in the past decade with the focus on observation and orbit dynamic models. First, the disclosed metadata of BDS satellites is presented and the contribution to BDS POD is addressed. The complete optical properties of the satellite bus as well as solar panels are derived based on the absorbed parameters as well the material properties. Secondly, the status and tracking capabilities of the L-band data from accessible ground networks are presented, while some low earth orbiter satellites with onboard BDS tracking capability are listed. The topological structure and measurement scheme of BDS Inter-Satellite-Link (ISL) data are described. After highlighting the progress on observation models as well as orbit perturbations for BDS, e.g., phase center corrections, satellite attitude, and solar radiation pressure, different POD strategies used for BDS are summarized. In addition, the urgent requirement for error modeling of the ISL data is emphasized based on the analysis of the observation noises, and the incompatible characteristics of orbit and clock derived with L-band and ISL data are illuminated and discussed. The further researches on the improvement of phase center calibration and orbit dynamic models, the refinement of ISL observation models, and the potential contribution of BDS to the estimation of geodetic parameters based on L-band or ISL data are identified. With this, it is promising that BDS can achieve better performance and provides vital contributions to the geodesy and navigation.

This article analyzes the BDS Phase III (BDS‐3) single point positioning (SPP) kinematic mode performance, under seven different dual‐frequency ionospheric free combinations (IFC), namely B1C/B2a, B1C/B3I, B1I/B2a, B1I/B2b, B1I/B3I, B2a/B3I, and B2b/B3I, using the observation data of 84 static multi‐GNSS experiment (MGEX) stations for 7 days. And the three coordinates are estimated each epoch as white noise. We have derived a detailed SPP model based on timing group delay (TGD) and inter‐signal correction (ISC) for IFC. Next, the BDS‐3's five signals are assessed in regard to carrier‐to‐noise density ratio (C/N0) and code multipath (MP), B2b exhibits the best data quality, with an average C/N0 of 44.51 dB‐Hz and the code MP value of 22.40 cm. The difference of C/N0 among BDS‐3 single frequency signal is not obvious. The average number of satellites for all seven different IFCs is between 8.45 and 8.56, and the average PDOP value is between 2.03 and 2.08. There is not much difference in the number of satellites and PDOP between different IFCs. The root mean square error (RMSE) was used to assess the SPP performance. The sorted order of SPP kinematic performance of dual‐frequency IFC, from best to worst, is B1C/B2a, B1C/B3I, B1I/B2a, B1I/B2b, B1I/B3I, B2a/B3I, and B2b/B3I. The results show that the SPP kinematic accuracy of the B1C/B2a dual‐frequency IFC in the three directions of north, east, and up (NEU) is 0.70, 0.69, and 1.68 m, respectively, and B2b/B3I IFC is 3.63, 3.72, and 8.49 m, respectively. B1C/B2a has a positioning accuracy ~5 times better than B2b/B3I. If there are multiple IFC, it is recommended to use B1C/B2a in SPP. Some stations have slightly larger errors in mid low latitude regions, while others have slightly smaller errors in high latitude regions. By comparing the B1C/B2a and B2b/B3I IFC SPP results of all 84 stations, it was found that the RMSE of all stations with B2b/B3I IFC was greater than that of B1C/B2a.

Solketal, a widely used glycerol-derived solvent, can be efficiently synthesized through heterogeneous catalysis, thus avoiding the significant product losses typically encountered with aqueous work-up in homogeneous catalysis. This study explores the catalytic synthesis of solketal using solid acid catalysts derived from recovered carbon blacks (rCBs), which are obtained through the pyrolysis of end-of-life tires. This was further converted into solid acid catalysts through the introduction of acidic functional groups using concentrated H2SO4 or 4-benzenediazonium sulfonate (BDS) as sulfonating agents. Additionally, post-pyrolytic rCB treated with glucose and subsequently sulfonated with sulfuric acid was also prepared. Comprehensive characterization of the initial and modified rCBs was performed using techniques such as elemental analysis, powder X-ray diffraction, thermogravimetric analysis, a back titration method, and both scanning and transmission electron microscopy, along with X-ray photoelectron spectroscopy. The catalytic performance of these samples was evaluated through the batch mode glycerol acetalization to produce solketal. The modified rCBs exhibited substantial catalytic activity, achieving high glycerol conversions (approximately 90%) and high solketal selectivity (around 95%) within 30 min at 40 °C. This notable activity was attributed to the presence of -SO3H groups on the surface of the functionalized rCBs. Reusability tests indicated that only rCBs modified with glucose demonstrated acceptable catalytic stability in subsequent acetalization cycles. The findings underscore the potential of utilizing end-of-life tires to produce effective acid catalysts for glycerol valorization processes.

This paper reviews the status of satellite navigation (as per 11 May 2020)—without claim for completeness—and discusses the various global navigation satellite systems, regional satellite navigation systems and satellite-based augmentation systems. Problems and challenges for delivering nowadays a safe and reliable navigation are discussed. New opportunities, perspectives and megatrends of satellite navigation are outlined. Some remarks are closing this paper emphasizing the great value of satellite navigation at present and in future.

The Crustal Movement Observation Network of China (CMONOC) has begun receiving BeiDou Navigation Satellite System (BDS) observations since 2015, and accumulated more than 2.5 years of data. BDS observations has been widely applied in many fields, and long-term continuous data provide a new strategy for the study of crustal deformation in China. This paper focuses on the evaluation of BDS positioning performance and its potential application on crustal deformation in CMONOC. According to the comparative analysis on multipath delay (MPD) and signal to noise ratio (SNR) between BDS and GPS data, the data quality of BDS is at the same level with GPS measurements in COMONC. The spatial distribution of BDS positioning accuracy evaluated as the root mean square (RMS) of daily residual position time series on horizontal component is latitude-dependent, declining with the increasing of station latitude, while the vertical one is randomly distributed in China. The mean RMS of BDS position residual time series is 7 mm and 22 mm on horizontal and vertical components, respectively, and annual periodicity in position time series can be identified by BDS data. In view of the accuracy of BDS positioning, there are no systematic differences between GPS and BDS results. Based on time series analysis with data volume being 2.5 years, the noise characteristics of BDS daily position time series is time-correlated and corresponding noise is white plus flicker noise model, and the derived mean RMS of the BDS velocities is 1.2, 1.5, and 4.1 mm/year on north, east, and up components, respectively. The imperfect performance of BDS positioning relative to GPS is likely attributed to the relatively low accuracy of BDS ephemeris, and the sparse amount of MEO satellites distribution in the BDS constellation. It is expectable to study crustal deformation in CMONOC by BDS with the gradual maturity of its constellation and the accumulation of observations.

Malicious botnets such as Mirai are a major threat to IoT networks regarding cyber security. The Botnet Defense System (BDS) is a network security system based on the concept of “fight fire with fire”, and it uses white-hat botnets to fight against malicious botnets. However, the existing white-hat Worm Launcher of the BDS decides the number of white-hat worms, but it does not consider the white-hat worms’ placement. This paper proposes a novel machine learning (ML)-based white-hat Worm Launcher for tactical response by zoning in the BDS. The concept of zoning is introduced to grasp the malicious botnet spread with bias over the IoT network. This enables the Launcher to divide the network into zones and make tactical responses for each zone. Three tactics for tactical responses for each zone are also proposed. Then, the BDS with the Launcher is modeled by using agent-oriented Petri nets, and the effect of the proposed Launcher is evaluated. The result shows that the proposed Launcher can reduce the number of infected IoT devices by about 30%.

Global Navigation Satellite System (GNSS) based velocity estimation is one of the most cost-effective and widely used methods in determining velocity in geodesy and transport applications. Highly accurate and reliable velocity measurements can be obtained by exploiting the raw Doppler, carrier phase, and pseudorange measurements with a GNSS receiver. There are several approaches to GNSS-based velocity determination. This paper investigates the characteristics of the approaches which are currently popular and applicable to the observations of Global Positioning System (GPS), BeiDou Navigation Satellite System (BDS), and their combination (GPS/BDS). Specifically, it evaluates the performance of the velocity estimated based on the Raw Doppler method, the Time-Differenced Pseudorange method, the Time-Differenced Carrier Phase method, and the Double-Differenced Carrier Phase method, in both static and dynamic modes and in open and urban scenarios. The experiments show that BDS has the advantages in delivering accurate velocity determinations over GPS in the Asia–Pacific region, and the effectiveness of the GPS/BDS in improving the overall accuracy of velocity determination in complex urban scenarios.

A common practice adopted for the pseudorange bias estimation and calibration assumes that Global Navigation Satellite System satellite-dependent pseudorange biases vary gently over time. Whereupon satellite pseudorange biases are routinely estimated and provided as the products with low temporal resolution, e.g., hourly or daily, by the agencies. The story sounds unquestionably perfect under the acquainted assumption. To validate the inadequacy of the above hypothesis we herein present an approach to the estimate the BeiDou Navigation Satellite System (BDS) pseudorange biases with high temporal resolution. Its feasibility, affecting factors, and necessity are discussed. Concretely, the Geometry-Free function models are first constructed to retrieve the linear combination of the pseudorange biases; then the pseudorange Observable-specific Signal Bias (OSB) values with respect to baseline frequencies ( e.g. , BDS C2I/C6I) are estimated along with the ionosphere modeling; subsequently, all multi-frequency pseudorange OSBs are determined by using the ionospheric information with constraint conditions; finally, the possible Differential Code Bias sets are attainable with the estimated pseudorange OSBs. Using the observation data of four months when the estimated BDS pseudorange biases are stable, their reliability is demonstrated with the stability at the level of sub-nanosecond and the BeiDou-3 Navigation Satellite System (BDS-3) values more stable than that of BeiDou-2 Navigation Satellite System (BDS-2). The comparison between the estimated pseudorange biases and the Chinese Academy of Sciences products reveals that the accuracy of the estimated pseudorange biases is 0.2–0.4 ns. Moreover, the large magnitude of the short-term pseudorange bias variation in the tens of nanoseconds for the BDS-2 and BDS-3 are found in years 2021 and 2022, which are affected by two types of the satellite flex power for the BDS-2 and BDS-3, respectively. We stress that it’s necessary to estimate the BDS pseudorange biases with high temporal resolution in the case of the satellite flex power and the products currently provided by the agencies cannot reflect the true quantity under the circumstance.

To meet the demands for the data combination with multiple space geodetic techniques at the observation level, we developed a new software platform with high extensibility and computation efficiency, named space Geodetic SpatioTemporal data Analysis and Research software (GSTAR). Most of the modules in the GSTAR are coded in C++ with object-oriented programming. The layered modular theory is adopted for the design of the software, and the antenna-based data architecture is proposed for users to construct personalized geodetic application scenarios easily. The initial performance of the GSTAR software is evaluated by processing the Global Navigation Satellite System (GNSS) data collected from 315 globally distributed stations over two and a half years. The accuracy of GNSS-based geodetic products is evaluated by comparing them with those released by International GNSS Service (IGS) Analysis Centers (AC). Taking the products released by European Space Agency (ESA) as reference, the Three-Dimension (3D) Root-Mean-Squares (RMS) of the orbit differences are 2.7/6.7/3.3/7.7/21.0 cm and the STandard Deviations (STD) of the clock differences are 19/48/16/32/25 ps for Global Positioning System (GPS), GLObal NAvigation Satellite System (GLONASS), Galileo navigation satellite system (Galileo), BeiDou Navigation Satellite System (BDS), Medium Earth Orbit (MEO), and BDS Inclined Geo-Synchronous Orbit (IGSO) satellites, respectively. The mean values of the X and Y components of the polar coordinate and the Length of Day (LOD) with respect to the International Earth Rotation and Reference Systems Service (IERS) 14 C04 products are -17.6 microarc-second (µas), 9.2 µas, and 14.0 µs/d. Compared to the IGS daily solution, the RMSs of the site position differences in the north/east/up direction are 1.6/1.5/3.9, 3.8/2.4/7.6, 2.5/2.4/7.9 and 2.7/2.3/7.4 mm for GPS-only, GLONASS-only, Galileo-only, and BDS-only solution, respectively. The RMSs of the differences of the tropospheric Zenith Path Delay (ZPD), the north gradients, and the east gradients are 5.8, 0.9, and 0.9 mm with respect to the IGS products. The X and Y components of the geocenter motion estimated from GPS-only, Galileo-only, and BDS-only observations well agree with IGS products, while the Z component values are much nosier where anomalous harmonics in GNSS draconitic year can be found. The accuracies of the above products calculated by the GSTAR are comparable with those from different IGS ACs. Compared to the precise scientific orbit products, the 3D RMS of the orbit differences for the two Gravity Recovery and Climate Experiment Follow-on (GRACE-FO) satellites is below 1.5 cm by conducting Precise Point Positioning with Ambiguity Resolution (PPP-AR). In addition, a series of rapid data processing algorithms are developed, and the operation speed of the GSTAR software is 5.6 times faster than that of the Positioning and Navigation Data Analyst (PANDA) software for the quad-system precise orbit determination procedure.

This contribution summarizes the strategy used by Wuhan University (WHU) to determine precise orbit and clock products for Multi-GNSS Experiment (MGEX) of the International GNSS Service (IGS). In particular, the satellite attitude, phase center corrections, solar radiation pressure model developed and used for BDS satellites are addressed. In addition, this contribution analyzes the orbit and clock quality of the quad-constellation products from MGEX Analysis Centers (ACs) for a common time period of 1 year (2014). With IGS final GPS and GLONASS products as the reference, Multi-GNSS products of WHU (indicated by WUM) show the best agreement among these products from all MGEX ACs in both accuracy and stability. 3D Day Boundary Discontinuities (DBDs) range from 8 to 27 cm for Galileo-IOV satellites among all ACs’ products, whereas WUM ones are the largest (about 26.2 cm). Among three types of BDS satellites, MEOs show the smallest DBDs from 10 to 27 cm, whereas the DBDs for all ACs products are at decimeter to meter level for GEOs and one to three decimeter for IGSOs, respectively. As to the satellite laser ranging (SLR) validation for Galileo-IOV satellites, the accuracy evaluated by SLR residuals is at the one decimeter level with the well-known systematic bias of about - 5  cm for all ACs. For BDS satellites, the accuracy could reach decimeter level, one decimeter level, and centimeter level for GEOs, IGSOs, and MEOs, respectively. However, there is a noticeable bias in GEO SLR residuals. In addition, systematic errors dependent on orbit angle related to mismodeled solar radiation pressure (SRP) are present for BDS GEOs and IGSOs. The results of Multi-GNSS combined kinematic PPP demonstrate that the best accuracy of position and fastest convergence speed have been achieved using WUM products, particularly in the Up direction. Furthermore, the accuracy of static BDS only PPP degrades when the BDS IGSO and MEO satellites switches to orbit-normal orientation, particularly for COM products, whereas the WUM show the slightest degradation.